With this spell, a dragon's breath weapon takes on the appearance and properties of the breath weapon of another type of dragon. For dragons who possess multiple breath weapon forms, only one is changed by the spell. In any case, the spell causes an actual change, not an illusion. Furthermore, the amount of damage caused by the altered breath weapon is the same as that of the dragon's true breath weapon; only the type, not the power, of the breath weapon is changed.
For example, a red dragon can use this spell to change its fiery breath weapon into a cloud of chlorine gas (as used by green dragons) but with a damage potential equal to its usual flame breath weapon. By using this spell, the dragon is able to harm creatures that are normally immune or resistant to its fiery breath (for example, another red dragon). Of course, a creature immune to chlorine gas or to dragon breath weapons in general is still unharmed by the attack.
Alter breath weapon can be ended prematurely with a successful dispel magic or similar effect or by silent will of the dragon. However, the spell does not permit the dragon to shift through multiple types of breath weapons; once a particular type of breath weapon is chosen for a particular casting of the spell, it cannot be changed, save to revert back to the dragon's true breath weapon, thus ending the spell. Likewise, multiple alter breath weapon spells cannot be in effect at the same time. If a second alter breath weapon is attempted before the first expires, both spells are immediately negated.